This invention relates generally to fans and blowers for delivering gases at specific flow rates and pressures and more particularly to an impeller or fan wheel of a diagonal-flow fan, the fan wheel being provided with blades each of the shape of a single-curvature surface which affords high performance of the fan substantially equivalent to that of the fan provided with blades each of an ideal shape of a twisted double-curvature surface.
In the fan wheel of an ordinary centrifugal fan the entrance edges and exit edges of the blades are respectively parallel to the rotational shaft axis. At the same time, when the fan wheel is viewed in its axial direction, each of its blades is arcuately curved as it extends toward the periphery of the fan wheel, and each blade has no twist with respect to the axial direction, and cross sections of the blades taken in parallel planes perpendicular to the axis appear to be superposed on each other. Thus, each blade has a single-curvature or developable curved surface.
Furthermore, most of the cross sections of these blades with single-curvature surface in an ordinary centrifugal fan have the shape of a single arc, or the shape of two arcs joined together. Accordingly, the fabrication of these blades is relatively simple. However, even in the case of a blade of this kind, a blade cross section shape in which the radius of the arc varies progressively along the chord length is close to the ideal shape from the viewpoint of fluid dynamics, but the fabrication of blades of such a shape is extremely difficult. For this reason, such blades have not as yet been reduced to practice except for centrifugal fans having blades of wing profiles (airfoil profiles) being manufactured in spite of this difficulty in order to utilize the advantages in efficiency and low noise level.
In contrast to a centrifugal fan as described above, a diagonal-flow fan has blades whose entrance edges and exit edges are not parallel to the rotational shaft axis, the radial distance from the shaft axis to each entrance edge varying progressively from one end of the entrance edge to the other, and furthermore, the radial distance from the shaft axis to each exit edge also varying progressively from one end of the exit edge to the other. In addition, each blade must be provided with a complicated double curvature which causes it to have a twist as viewed in the shaft axial direction. These and other features of diagonal-flow fans will be described in detail hereinafter, particularly in comparison with a centrifugal fan.
Theoretically, a diagonal-flow fan should have excellent performance but has not been reduced to practical use because of certain difficulties as will be described hereinafter.